Step 1: Prepare the Spice Cake

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, ginger, nutmeg, cloves, and allspice.
3. Cream Butter and Sugars: In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar until light and fluffy.
4. Add Eggs and Vanilla: Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
5. Combine Wet and Dry Ingredients: Alternately add the dry ingredients and buttermilk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
6. Bake: Divide the batter evenly between the prepared cake pans and smooth the tops. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
7. Cool: Let the cakes cool in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

Step 2: Make the Eggnog Buttercream

1. Beat Butter: In a large bowl, beat the softened butter until smooth and creamy.
2. Add Powdered Sugar and Eggnog: Gradually add the powdered sugar, beating well after each addition. Add the eggnog and vanilla extract, mixing until the frosting is light and fluffy. If the frosting is too thick, add a little more eggnog, one tablespoon at a time, until it reaches the desired consistency.
3. Add Nutmeg (Optional): For extra holiday flavor, add ground nutmeg to the frosting and mix well.

Step 3: Assemble the Cake

1. Frost the Cake: Place one layer of the spice cake on a serving plate and spread a layer of eggnog buttercream on top. Add the second layer of cake and frost the top and sides of the cake with the remaining buttercream.
2. Decorate: Dust with a little extra ground nutmeg or add holiday sprinkles for a festive look.